An acoustic sensor (or microphone) is an apparatus that converts voice into an electrical signal. Recently, as the development of small wired and wireless equipments has been accelerated, a size of the acoustic sensor has become smaller and smaller. Therefore, an acoustic sensor using an MEMS technology has been developed recently.
The acoustic sensor is largely classified into a piezo-type and a condenser-type. The piezo-type uses a piezo effect that generates a potential difference across a piezo substance when a physical pressure is applied to the piezo substance and is a type that converts a pressure of a voice signal into an electrical signal. The piezo-type may have many limitations in applications due to non-uniform characteristics of a low-band and voice-band frequency. The condenser-type uses a principle of a condenser in which two electrodes face each other, wherein one electrode of an acoustic sensor is fixed and the other electrode thereof serves as a diaphragm. This is a type in which a capacitance between the electrodes is changed when the diaphragm is vibrated according to the pressure of the voice signal and thus, the accumulated charges are changed, thereby making current flow. The condenser type has excellent stability and frequency characteristics. Due to the frequency characteristics, the condenser type acoustic sensor has been mainly used.